To facilitate an understanding of the many advantages of the invention and to provide for ease of description, the invention will be specifically described hereinafter with reference to providing protective and decorative coatings on exterior automotive panels, but it should be understood that the automobile is only one of many different types of products in whose manufacture the invention can be beneficially applied.
As pointed out in an article by Alan J. Backhouse entitled "Routes To Low Pollution Glamour Metallic Auomotive Finishes", Journal of Coatings Technology, Vol. 54, No. 693, pages 83-90, October 1982, there is a growing need to reduce the amount of atmospheric pollution caused by solvents emitted during industrial paining processes. Many different approaches to meeting this need have been proposed. For example, efforts have been made to replace the solvent-based paints typically used for automobiles with water-based paints. As another alternative, work has been done to facilitate the use of high solids formulations which will result in proportionately less emission of organic solvents. However, the application of automotive finishes is a highly demanding art because of the extremely high quality of the surface finish required and because of the very common application of metallic finishes to provide what Backhouse refers to as "high stylistic effects". Accordingly, past efforts to replace the low viscosity, low-solids-content paint formulations conventionally used in spray painting operations in the automotive industry have met with only very limited success.
A much more promising approach to solving the problem is to eliminate entirely the need for spray painting operations to provide the necessary protective and decorative coating on exterior automotive panels. Elimination of spray painting, or a substantial reduction in the extent of its use, would not only be environmentally beneficial in reducing atmospheric pollution, but would be extremely beneficial from a cost savings standpoint in that spray painting operations are wasteful of the paint to such an extent that more than half of the paint may be lost as waste material. A means for achieving such goal exists through the use of a pre-formed thermoplastic sheet material which can be glued or otherwise securely bonded to the panel to provide the protective and decorative coating. Such techniques are well known and widely used in industry, and have been utilized for such purposes as interior automobile panels as described, for example, in U.S. Pat. No. 3,551,232 issued Dec. 29, 1970. This art is summarized in the aforesaid patent in the following words:
"It is present day conventional practice to make structural members consisting of a relatively rigid substrate to which is bonded as a surface or cover layer a synthetic resin sheet. The surface layer may be smooth or embossed and may be suitably colored to provide a desired decorative effect. The substrate may be formed of a relatively rigid synthetic resin, such as polystyrene, or sheet metal, and the surface layer and the substrate are bonded together to form a laminate. Such structures may be used for a variety of purposes such as interior automobile panels, glove compartment doors, and the like. PA1 A convenient and economical method for making such articles involves the application of a suitable adhesive to the surface of the substrate and then vacuum forming the decorative cover layer over the adhesive layer of the substrate".
The objective of U.S. Pat. No. 3,551,232 is to overcome the problems of bubbling and blistering of the resin sheet that tend to occur in the vacuum-forming process. It achieves this by use of an adhesive containing an inert particulate filler which minimizes the entrapment of air.
To employ a process of the type described in U.S. Pat. No. 3,551,232 with exterior automotive panels presents a much greater challenge. The surface appearance of such panels is of critical importance, so that it is necessary not only to avoid such problems as bubbling or blistering caused by entrapped air, but to provide a protective and decorative coating that will equal or exceed in many respects, the quality of a spray-painted surface and that is equally capable of providing the "high stylistic effects" that are of growing importance in the automotive marketplace. Furthermore, exterior automotive panels present a particular problem in view of the great difficulty of smoothly adhering a flexible sheet material to a substrate which may possess complex curvature and the even greater difficulty of doing so while maintaining over the entire surface of the panel a uniform degree of the color intensity exhibited by the coating.
Heretofore, efforts have been made to produce a flexible and stretchable sheet material having these capabilities through the application of one or more paint layers to the surface of a thermoformable polymeric support. However, such efforts have, prior to this invention, been unsuccessful in that the sheet material has suffered from various defects which have rendered its commercial use impractical. Thus, for example, prior efforts have resulted in products which exhibit numerous coating defects and in which the paint layer does not have the high degree of uniformity which permits the sheet material to undergo the severe stresses of the thermoforming process and yet provide a product which meets the exacting quality standards of an exterior automotive finish.
It is toward the objective of providing a flexible and stretchable sheet material which is capable of being stretched to conform to a three-dimensional substrate, such as an exterior automotive panel, to provide a high quality defect-free protective and decorative coating of uniformly attractive appearance, that the present invention is directed.